The requirements for the signal quality of modulators, for example in transmitting devices, become more stringent as the need for high data rates and increasing mobility grows. The modern mobile radio standards such as universal mobile telecommunications system (UMTS), wideband code division multiple access (WCDMA), global system for mobile communication (GSM), enhanced data rates for GSM evolution (EDGE), Bluetooth medium data rate or Wireless local area network (WLAN) according to 802.11a/b/g require special modulation types for data transmission, which modulate, for example, both the phase and the amplitude of a carrier signal at the same time.
Simultaneous amplitude and phase modulation make it possible to achieve higher data transmission rates and thus better bandwidth efficiency. The mobile radio standards mentioned above envisage, for example, the use of quadrature phase shift keying (QPSK), eight phase shift keying (8-PSK) or quadrature amplitude modulation (QAM) as modulation types for the data transmission.
Depending on the selected application for the individual mobile radio standards, these high-quality modulation types are used not only for data transmission from a base station to a mobile communication appliance but also from the mobile communication appliance to the base station.
The modulation types which are used for modern mobile radio standards are particularly sensitive to possible distortion which is produced by various components in a transmission path. Distortion such as this in the transmission path leads to changes in the phase and amplitude of a carrier signal. This results in data errors in the transmitted signal.
In order to suppress the distortion, it can be desirable to provide components in the modulator arrangement which have a highly linear transmission characteristic. In this case, the expression, ‘linearity of transmission characteristic’, means a transmission response of an element within the modulator arrangement which essentially produces an output signal that is proportional to the respective input signal. Accordingly, circuits whose transmission responses show non-linear behavior produce an output signal which is not proportional to the respective input signal. Such a non-linearity can lead to data errors within the transmitted signal.
Elements having a non-linear transmission characteristic can, for example, be amplifiers provided in the transmission path. By way of example, in the case of power amplifiers, a high degree of linearity in the output signal is achieved by operating the power amplifiers considerably below their maximum achievable output power level. This is referred to as operation in a linear area of the characteristic.
However, operation of a power amplifier in this way can lead to a high quiescent current being drawn, thus increasing the overall power consumption. Particularly in the case of mobile communication appliances, an operating time is reduced by a greater current, which is governed by the capacity of rechargeable batteries used in the mobile communication appliance.
Modern mobile communication appliances usually attempt to reach a compromise between the current that is drawn and the linearity of the individual active switching elements in a transmission path. This can be achieved by suitable circuitry. By way of example, it is possible to reduce the current that is drawn by choice of suitable biasing, the adjustment of the operating points, and by a suitable load impedance of the output of the components having a non-linear characteristic.
In order to further improve a transmission response of the overall transmission path and in order to reduce possible data errors, it is normal in modern transmitting devices to additionally predistort the input signal.
In the example of predistortion, an improvement in the signal quality is achieved by supplying a distorted signal to the amplifier or to the component with the non-linear characteristic, respectively. The distortion is in this example chosen such that the distortion caused by the transmission response is accurately compensated for. It is then once again possible to tap off a signal which is approximately proportional to the input signal at the output of the amplifier or of the component with a non-linear characteristic.
The use of a predistortion, however, causes a higher power consumption in a modulator arrangement or transmitting device. It therefore can be desirable to find a tradeoff between a power efficient operating range of amplifiers and a use of predistortion in the modulator arrangement.